Rotor blade assembly

ABSTRACT

Rotor blade and disk structures enabling the disassembly and reassembly of a single rotor blade are disclosed. The concepts are particularly suited to configurations employing interlocking mid-span shrouds which inhibit the withdrawal of a single blade from a disk. 
     In one specific geometry the rotor blade root section (20) and the disk attachment slot (24) are contoured to provide increasing clearance between the bottom of the blade root section and the attachment slot as the blade is withdrawn axially from the slot. Rocking of the blade within the provided clearance enables withdrawal of the blade around the shroud segments (30,32) and airfoil sections (16) of the adjacent blades (12).

DESCRIPTION TECHNICAL FIELD

This invention relates to axial flow rotary machines, and moreparticularly to the rotor blades of such machines.

The concepts were developed in the aircraft gas turbine engine industryto enable the disassembly of single fan blades from the fan sections ofturbofan engines, but have wider applicability both within that industryand others as well.

BACKGROUND ART

Modern aircraft gas turbines are of the turbofan type having large rotorblades usually at the forward end of the engine. The blades are termed"fan blades" and are utilized to accelerate air directed thereover in amanner much the same as a propeller.

Being located at the forward end of the engine, the blades aresusceptible to foreign object damage as a result of debris picked upalong the airport runway and ingested into the engine. Large birds arenot an infrequent cause of fan damage. Damaged blades must be replacedto restore aerodynamic efficiency to the fan and to prevent destructiverotor imbalance.

Fan blade roots are conventionally formed to a dovetail cross-sectiongeometry and extend from fore to aft across the rim of a supportingdisk. The root attachments are usually canted circumferentially withrespect to the centerline or axis of the engine. Blades of long spanhave one or more shrouds at an intermediate position along the span ofthe blades or at the blade tips. Such a shroud is formed of elementsextending laterally from the pressure and suction sides of the bladesinto opposing relationship with the shroud elements of adjacent blades.In combination, the shroud elements form an annular ring when viewed ina direction along the axis of the engine. The plane of opposingrelationship between adjacent shroud elements is nonparallel to the axisof the blade attachment, thereby locking each individual blade into theassembly.

To avoid the expense and necessity of removing all blades of a rotorstage in order to replace a single blade, scientists and engineers inthe industry are searching for new blade concepts and techniques of thisassembly.

DISCLOSURE OF INVENTION

According to the present invention, the disassembly of a single shroudedblade from an axial flow rotary machine is enabled by providing awedge-shaped protrusion at the base of the disk attachment slot suchthat the blade can be rocked about the wedge-shaped protrusion uponpartial withdrawal of the blade from the slot to effect disengagement ofthe blade from the shrouds of adjacent blades.

Primary features of the present invention are the wedge-shapedprotrusions extending outwardly from the base of each disk attachmentslot and the tapered surfaces at the bottoms of the root sections of thecorresponding blades. Partial withdrawal of the root section of a bladefrom the corresponding attachment slot increases the radial clearancetherebetween and enables rocking of the blade about the wedge-shapedprotrusion.

A principal advantage of the present invention is the enableddisassembly of a single blade from the rotor assembly. In an aircraftinstallation, removal of a fan blade can be accomplished in situ withoutremoving the engine from the aircraft. Such disassembly is accomplishedwithout recourse to increased part clearances in the installedcondition. Rocking the blade in a first direction about the wedge-shapedprotrusion enables partial withdrawal notwithstanding the nonparallelrelationship of the axis of the attachment slot and the planes alongwhich adjacent shrouds abut. Rocking of the blade in a second directionpermits circumferential displacement of the blade shroud about thetrailing edge of the adjacent blade, such that the blade may becompletely withdrawn from the attachment slot.

The forgoing, and other features and advantages of the presentinvention, will become more apparent from the following description andthe accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified front elevation view of a portion of the fanassembly of a turbofan gas turbine engine;

FIG. 2 is an exploded perspective view illustrating the wedge againstwhich the blade root seats;

FIG. 3 is a view taken in the direction 3--3 as shown in FIG. 1;

FIG. 4 is a sectional view taken along the line 4--4 as shown in FIG. 1;

FIG. 5 is an enlarged view of the fan blade root illustrating theability of a partially removed blade to tilt about the wedge at the baseof the attachment slot;

FIG. 6 is a perspective view illustrating partial withdrawal of a singlerotor blade from the assembly;

FIG. 7 is a perspective view illustrating the sequential partialwithdrawal of two adjacent rotor blades with the shroud of the mostforwardly withdrawn blade free of interference with the shroud of theadjacent blade; and

FIG. 8 is a perspective view illustrating circumferential displacementof the most forwardly withdrawn blade to a position at which the shroudof the blade is free of interference with the airfoil section of theadjacent blade.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention is described with respectto the fan section of a turbofan gas turbine engine. A portion of a fanrotor assembly is illustrated in the FIG. 2 front elevation view. Aplurality of rotor blades 12 extend outwardly from the periphery of arotor disk 14. Each rotor blade has an airfoil section 16, a platformsection 18 and a root section 20. Each platform section defines aportion of the inner wall of the flowpath 22 for working medium gases.Each root section engages a corresponding attachment slot 24 at theperiphery of the disk. The airfoil section of each blade has a suctionside 26 and a pressure side 28. Shroud segments 30 and 32 extendlaterally from the suction and pressure sides of the blade respectivelyinto opposing relationship with the shroud segments of adjacent blades.In composite the shroud segments of adjacent blades form an annular ringextending circumferentially around the engine. The shroud illustrated ispositioned at a midpoint along the span of the blade and is termed a"mid-span" shroud. The spanwise location of the shroud varies withengine design. Multiple shrouds are employed in some embodiments and inothers a shroud is positioned at the tips of the blades.

The shape of each blade root section and the geometry of thecorresponding attachment slot are shown in the FIG. 2 exploded,perspective view. The attachment geometry is of the generic type knownas a "dovetail root". The geometry is modified, however, from theconventional form in that the bottom surface 34 of the blade root istapered from the leading edge 36 to the trailing edge 38, forming a rootof decreasing cross section. The attachment slot 24 has a wedge-shapedprotrusion 40 extending from the center portion of the bottom of theattachment slot from the front side 42 to the rear side 44 of the disk.The top surface 46 of the wedge-shaped protrusion and the bottom surfaceof the blade root section are matched to an angle φ measured withrespect to a plane parallel to the engine's centerline. The root sectionof the rotor blade and the attachment slot of the disk are contoured toprovide increasing clearance between the bottom of the blade rootsection and the attachment slot as the blade is axially withdrawn fromthe slot. The blades 12 are trapped axially on the disk 14 in theassembled mode by conventional means not specifically illustrated.

Concepts integrated into the described apparatus enable the in situdisassembly and reassembly of a single rotor blade, such as might berequired after operation in an engine on account of foreign objectdamage to the blades. In conventional construction such in situdisassembly and reassembly of a single blade is inhibited bycircumferential interference between adjacent shroud segments and axialinterference of the shroud segments with forwardly extending portions ofthe adjacent airfoil sections. As is viewable in FIG. 3, the axis A ofthe blade root attachment may not be parallel to the plane ofintersection B with the result that the blades held in axial position onthe disk are interlocked. Even without direct restrain each blade isincapable of axial displacement greater than the distance Y between thesuction side shroud segment 30 of that blade and the airfoil section 16of the adjacent blade.

The revised root section and attachment slot geometry of the presentinvention enables combined axial and circumferential blade displacementfreeing the shroud segments of the blade to be withdrawn from both theadjacent shroud segments and the airfoid section of the adjacent blade.The FIG. 4 illustrates increased clearance between the bottom surface 34of the blade root section and the top surface 46 of the wedge-shapedprotrusion 40 at the base of the attachment slot. In the installedcondition the clearance is of a magnitude C₁ ; in the partial withdrawncondition as represented by the dotted configuration, the clearance isof a magnitude C₂.

The increased clearance upon partial axis withdrawal enables the bladeto initially rock about the wedge-shaped protrusion 40 in accommodationof the differential alignment of the blade root axis A and the plane ofintersection B of the adjacent shroud segments and to subsequently rockabout the wedge-shaped protrusion 40 to rotate the suction side shroudsegment circumferentially free of the adjacent airfoil section. Theability of the blade to rock about the wedge-shaped protrusion in apartially withdrawn condition is viewable in FIG. 5.

The sequential steps for withdrawing a single rotor blade from a rotordisk in one embodiment are shown in FIGS. 6-8. Although not shown in theinstalled condition on a gas turbine engine, the principles ofdisassembly and reassembly of the blades 12 from the rotor disk 14 areidentical. Direct axial restraint trapping the blades on the disk in thefully assembled mode has been removed.

In FIG. 6 one of the blades 12A is adjusted forwardly on the disk 14until the suction side shroud segment 30A abuts the airfoil section 16of the adjacent blade. The blade rocks slightly in the attachment slotin a clockwise direction to accommodate the angular mismatch of the rootaxis and the shroud segment plane of the intersection. In FIG. 7 asecond rotor blade 12B is adjusted forwardly in the disk until thesuction side shroud segment 30B of the second blade abuts the airfoilsection 16A of the adjacent blade. In the embodiment illustrated thepressure side shroud segment 32B is free of circumferential interferencewith the suction side shroud segment 30C of the adjacent blade 12C. Thenumber of blades 12 which must be forwardly adjusted depends upon theparticular embodiment including such factors as the width of the shroudsegments and the geometry of the adjacent airfoil section in thevicinity of the shroud.

In FIG. 8 the blade 12B is rotated in the attachment slot in acounterclockwise direction to a position at which the suction sidesegment 30B of the blade is free to move axially past the airfoilsection 16A of the adjacent blade. In this position full withdrawal of asingle blade from the disk can be made. Reassembly of the single bladeis made by a procedure reverse to that by which disassembly is made.

Although the invention has been shown and described with respect todetailed embodiments thereof, it should be understood by those skilledin the art that various changes in form and detail thereof may be madewithout departing from the spirit and the scope of the claimedinvention.

We claim: .[.
 1. For an axial flow gas turbine engine having a rotorassembly of a type including an array of rotor blades extendingoutwardly from the periphery of a rotor disk and wherein the airfoilsection of each of said rotor blades has shroud segments extendinglaterally from the suction and pressure sides thereof into opposingrelationship with corresponding shroud segments of the adjacent rotorblades, the improvement comprising:means for attaching each rotor bladeto a corresponding attachment slot at the periphery of the rotor diskwherein the root section of the blade and the attachment slot of thedisk are contoured to provide increasing clearance between the bottom ofthe blade root section and the attachment slot as the blade is withdrawnaxially from the slot..].
 2. .[.The invention according to claim 1.]..Iadd.For an axial flow gas turbine engine having a rotor assembly of atype including an array of rotor blades extending outwardly from theperiphery of a rotor disk and wherein the airfoil section of each ofsaid rotor blades has shroud segments extending laterally from thesuction and pressure sides thereof into opposing relationship withcorresponding shroud segments of the adjacent rotor blades, theimprovement comprising:means for attaching each rotor blade to acorresponding attachment slot at the periphery of the rotor disk whereinthe root section of the blade is a dovetail-shaped root section, andwherein said root section and the attachment slot of the disk arecontoured to provide increasing clearance between the bottom of theblade root section and the attachment slot as the blade is withdrawnaxially from the slot and are contoured such that the blade root sectionis able to rock within such clearance, .Iaddend.wherein the diskattachment slot has a wedge-shaped protrusion extending from the centerportion of the bottom of the attachment slot and tapered to increasingcross section from the front to the rear sides of the disk, and whereinthe rotor blade root section has a bottom surface of taper correspondingto that of the wedge-shaped protrusion of the attachment slot.